Acute promyelocytic leukemia (APL) is often associated with a severe bleeding diathesis. There has been much debate amongst hematologists as to the mechanisms behind this. Although probably multiple factors are involved, accumulating evidence in the literature supports that a primary fibrinolytic process plays a major role. Annexin II (Ann II), a newly identified endothelial cell surface protein, is a co-receptor for plasminogen (PLG) and its activator, tissue plasminogen activator (t-PA), and increases the catalytic efficiency of plasmin generation up to 60 fold over baseline in a purified system. Most APL cells have a unique cytogenetic abnormality, a 15;17 balanced translocation. Preliminary data suggest that the t(15;17) positive APL cell line, NB4, expresses Ann II both on its surface and at the mRNA level. Furthermore, Ann II expression in NB4 cells can be down regulated by all-trans retinoic acid (ATRA), a novel treatment for APL. Based upon these preliminary data, this project will test the hypothesis that Ann II contributes to the bleeding disorder of APL by enhancing fibrinolysis. The research will focus on the expression of Ann II in APL and other leukemia cells, and how Ann II is affected by the t(15;17) at the transcriptional level using ribonuclease protection assays, specific transfection experiments, and nuclear run on experiments; and at the protein level using western blotting, fluorescent activated cell sorting, indirect immunofluorescence microscopy, specific functional assays, and radiolabelled binding assays. In addition, cells from leukemia patients will be evaluated for Ann II expression and correlated with the degree of coagulopathy. Confirmation of the role of Ann II may lead to important conclusions regarding the management of bleeding in these patients. Since Ann II appears to be dysregulated in this disorder (perhaps secondary to the translocation), this project may also provide fundamental new knowledge as to control of Ann II expression at the transcriptional, translational and processing levels. This could have far-reaching implications for the diagnosis and treatment of thrombotic vascular disease in a variety of settings and perhaps in the normal functioning of mature neutrophils.